1. Field of the Invention
The present invention relates generally to a knife and more particularly pertains to a knife wherein the blade is automatically retracted when the knife is not being used.
2. Description of the Related Art
It is generally recognized that a utility knife presents a danger to the operator due to the sharpness of the blade in the knife protruding from the handle. One approach for reducing the danger has been to provide knives which permit a manual retraction of the blade into the handle of the knives. A common form of such a knife includes a blade which has a thumb-actuated slide mechanism to extend and retract the blade.
U.S. Pat. No. 4,393,587 to Kloosterman, the disclosure of which is incorporated by reference, discloses a utility knife having an automatically retracting blade. In this patent, there is an outer cylindrical handle, an inner cylindrical blade cover, and the blade holder. The blade holder is attached to the outer cylindrical handle which is spring-biased fully from the inner cylindrical blade cover. When a compressive force is exerted on the outer handle, the blade is extended. While this invention is an improvement of the art, it has a significant drawback. Any time a compressive force is exerted on the handle, the blade will extend. Therefore, if the operator inadvertently presses the knife against a part of his body, the blade may extend and cause injury. A utility knife is needed in which the blade may not be inadvertently extended.
In addition, the knife of Kloosterman has another significant drawback. The blade guide and handle are oriented with respect to one another such that the operator""s hand must be in an uncomfortable position to operate the knife. In the device of Kloosterman, the handle and a line perpendicular to the blade guide are at an angle of approximately 45 degrees with respect to one another. In this position, the knife must be held with the thumb and forefinger adjacent the blade. This results in an uncomfortable hand position. A utility knife is also needed which allows the user to operate the knife in a ergonomically desirable position.
The present invention overcomes the deficiencies of utility knives of the past. The invention includes an automatically retracting blade on a utility knife which requires a dual action of the hand to extend the blade. The utility knife of the present invention also is designed such that the operator may hold the knife in the ergonomically optimal position.
The present invention provides a knife with a replaceable blade which is spring-biased in the retracted position. An outer generally cylindrical housing is provided which acts as a handle for the knife. An inner, generally cylindrical housing is provided which is slidably mounted within the outer housing. The knife is designed such that the blade can be extended only by simultaneously exerting a downward force and turning the outer handle with respect to the inner housing. The handle has an internal shoulder member which extends toward the center of the housing. The inner housing also has an internal shoulder member which extends toward the center of the housing. The shoulder of the inner housing has an axial bore.
A blade support member is adapted to hold a blade at a first end and at a second end extends through the bore and the shoulder member of the inner housing. The second end of the blade support member is trapped between the shoulder member of the outer housing and a second shoulder member attached to the outer housing. The space between the inner housing shoulder member and the outer housing shoulder member contains a spring adapted to telescopically bias the outer housing away from the inner housing. The outer housing has a tab affixed to the inside wall thereof. The inner housing has an angled slot in its wall adapted to receive the tab. When the blade is fully retracted, and the outer housing is rotated with respect to the inner housing, the tab rests on the shoulder at the top of the inner housing. In this position, the slot is angularly offset from the tab such that a straight downward pressure on the outer housing causes the tab to engage the shoulder and will not allow the blade to extend. When the outer housing is counter-rotated and the pressure is overcome, the tab enters the slot and the blade support member is forced downward with respect to the inner housing displacing the blade downwards. A blade cover provided at the bottom of the inner housing includes a bottom surface with a slot therein. As the blade is displaced, the cutting tip of the blade extends through the slot at the bottom of the blade cover such that the operator may use the blade for cutting objects. When the blade is removed from the object to be cut, and downward pressure on the handle is relieved, the spring forces the handle to move telescopically with respect to the inner housing and causes the blade to be retracted within the blade cover.
The outer housing has a longitudinal axis which is between 1 degree and 35 degrees from the line perpendicular to the bottom surface of the blade cover; preferably the angle is about 15 degrees. In this position, the operator holds the handle with his thumb and forefinger at the end of the handle opposite the blade. In prior art designs, the angle between the perpendicular to the cutting surface and the handle is such that the handle is oriented on the side of the perpendicular opposite the blade and adjacent the operator. In the present invention, the handle is on the same side of the perpendicular as the blade and opposite the operator. In other words, in the present invention the handle is angled away from the operator. The prior art designs are such that the handle is angled toward the operator. This design of the present invention enables the operator to hold the knife in the thumb-up position as shown in FIG. 8. In this position, the wrist of the operator is at a natural and ergonomically optimal position. In knives of the prior art, the knife is held such that the thumb and forefinger are at the end of the handle adjacent the blade. In this position, the wrist of the operator is in an unnatural position and over time, may result in repetitive stress injury such as carpal tunnel syndrome.